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Millimeter-wave spectrum of isocyanamide, H2N-NC
Volume
8 1, number
2
MILLIMETER-WAVE
E&hard
SCH&ER,
CHEMICAL
PHYSICS
SPECTRUM OF ISOCYANAMIDE,
LETTERS
15 July
1981
HzN-NC.
Manfred WINNEWlSSER
Physikalisch-Chemisches Institut. Justus-Liebig-UniversZt
Giessen. D-6300
Giessen, West Germany
and J@rn Johs. CHRISTIANSEN Department of Chemistry, The Royal Danish School of Educational Studies. DK-2400 Received
n
6 March 1981;
in final form 19 March 1981
The a-type rotatronal spectrum of isocyanamide was observed of the hydrolysis products of diazomethj llithmm was employed signed to molecules in theground vibrational state (NH&wersion (NHZ-inversion state 03.
l_ Introduction
Several theoretical studies of the various diazomethane Isomers [l-3] have predicted that cyanamide (I) is the most stable isomer followed by isocyanamide (II) and diazomethane (III). H,N-CN I
H,N-NC II
Copenhagen NV. Denmark
H,C=N=N III
HC=N=NH IV
has been the subject of numerous spectroscopic investigations [4-121 and its molecular structure and H2N-inversion motion have been analyzed_ Millimeter-wave measurements [ 111 confirmed the identification of cyanamide as a constituent of the interstellar molecular cloud in Sgr B2 [ 13,14]_ Isocyanamide was prepared by Milller and Ludsteck _[ 151 but was mistaken for nit&mine (IV)_ However, the chemical reactions of the compound which were carried out later provided strong evidence for isocyanamide [ 16-18]_ The isocyanamide structure was also supported by infrared studies [ 171 in solution and ti the solid phase [ 19,20]_ NMR spectra [17,20] and the strong isonitrile smell of the hydrolysis products of LiCHN, ) observed by MIiller and Ludsteck [ 151, further supported structure II. The barrier to isomerization between the structures Cyanamide
in the frequency range 147-300 GHz. Ether extraction to isolate the isocyanamide. 53 rotational lines were asstateO+) and m the lowest excited vtbrational state
I and II was reported by a recent ab initio calculation to be of the order of 46 kcal/mol which should stabilize the isocyanamide molecule at moderate temperatures [3] _Another pathway of isomerization from structure IV might be possible but has not been calculated_ In this paper we describe our results in searching for the isocyanamide millimerer-wave rotational spectrum and its unambiguous assignment. During the course of this work the millimeter-wave spectrum of diazomethane was also measured in the entire millimeter-wave range. The diazomethane data will be reported at a later date [21] _
2. Experimental procedures Isocyanamide was prepared dccordiug to the method described by Miiller and Ruqdel [22]. A solution of N20 in ether at a temperature of -85°C was treated with an excess of methyl lithium. The precipitate was separated by filtration and afterwards suspended in ether. This suspension was hydrolyzed with an aqueous solution of KH2P04 _Isocyanamide was extracted with ether, dried and concentrated in high vacuum. At a temperature of -30°C the sample was pumped for several hours until the vapour pressure remained below 2 pbar.
380 0 009-26
14/8 1 /OOOO-0000/S
02-50 0 North-Holland Publishing Company
CHEMICAL PHYSICS LETTERS
Volume 81, number 2
The search for the millimeter-wave rotational spectrum was performed with a newly built submillimeter-wave spectrometer designed for the study of unstable species 1211. The absorption cell consisted of a Pyrex free-space cell with an inner diameter of 10 cm and a length of 150 cm. The spectrum was measured using frequency multiplication from a klystron fundamental frequency between 60 and 80 GHz and employing source modulation and second-harmonic phase-sensitive detection of the modulation frequency of 3.5 kHz. Fig. 1 shows a survey spectrum of one transition recorded with this system. The absorption cell was fdled with isocyanamide vapour by keeping the sample at a temperature of -25°C and evaporating it continuously into the cell. The half-life of isocyanamide in the absorption cell at a total pressure of 10 pbar is = 15 min which is comparable to the stability observed for diazomethane under similar conditions. The partial pressure of isocyanamide probably did not exceed 4 pbar. Cyanamide has been identified as a by-product of the abovementioned synthesis or as a decomposition product via Its known millimeter-wave spectrum [ 1 l] _ Diazomethane was
15 July 1981
not observed as a decomposition product of isocyanamide. It should be mentioned that in a gold-plated Stark cell of a conventional microwave spectrometer, isocyanamide could not be observed due to rapid decomposition.
3. Spectra and preliminary analysis The observed transitions found in the millimeterwave region showed the typical pattern of a-type Rbranch lines with almost equidistant Ka = 0 lines for successive J values and widely spread K, = 1 doublets due to the slight asymmetry of the molecule (see table 1). The observed i (B + C) and B - C values indicated strongly that the observed lmes arise from isocyanamide. The rotational constants are in very good agreement with the latest ab initio calculations reported by Vincent and Dykstra [3 J _In particular, they predicted the B - C value to be 233 MHz, and the average B - C for the O+ and O- states found from our results is 23 1.7 MHZ. The ab initio calculations [3] provided the further
H,N-NC
850
233)900
950
234000
MHZ
Fig. l_ Part of the observed millimeter-wave spectrum of isocyanamide at 234 GHz with the typical Ka pattern for both inversion states O+ and O- exh~biiing the effect of nuclear spin statistics.
381
Volume81,numberZ
CHEMICALPHYSICSLETTERS
lSJuly1951
Table 1 a-typeR-branchrotationaltransitionsofisocyanamide,HtN-NC --Transttion JW,, Kc+-JCK,,
O'inversion statefrequencies(MHz)
O-inversionstate frequencies(MHz)
&I observed
calculated --
obs -talc. observed &Hz)
calculated
212825969(18)a)
212819996 (17)
2(0,2)-l(O,l) 2(1,1)-l(1,O) 2(1.2)-l(l,l)
42564.9183(36) 42795.3019(48) 42331.4330(51)
42563.7244(33) 42793.6940(49) 42330.6673(46)
3(0,3)-2(0,2) 3(1,2)-2(1,1) 3(1,3)-2(1,2) 3(2,1)-2(2,0) 3(2,2)-2(2,1)
63846.6890(52) 641925325(70) 63496.7506(74) 638376351(44) 63837.0469(44)
63844.8995(48) 64190.1214(71) 63495.6022(67) 63835.8875(35) 63835.3012(35)
4(0,4)-3(0,3) 4(1,3)--3(J,2) 4(1,4)-3(1,3) 4(2,2)-3(2,1) 4(2.3)-3(2,2) w--3(3,1)
85127.6334(66) 85589.2586(89) 84661.5894(95) 851168311(56) 85115.3601\56) 85098_4304(65)
85125.2503(61) 85586_0451(90) 84660.0588(86) 85114_4993(45) 85113.0336(445) 85096.1777(55)
5(0,5)-4(0,4) 5(1,4)-4(1,3) 5(1,5)-4(1,4) 5(2,3)-4(2,2) 5(2,4)-4(2,3) 5(3,3)-4(3,2) 5(4,2)-4(4,1)
106407.4763(77) 106985.3119(104) 105825.7901(112) 106396_0134(67) 106393.0729(66) 106372.2327(77) 106340 8408(145)
106404.5020(72) 1069812972(106) 105823.8777(101) 106393.0960(53) 106390.1648(53) 106369.4166(65) 106338.1529(131)
6(0,6)--5(O,5) 6(1,5)--5(1,4) 6(L,6)-5(1,5) 6(2,4)-S(2,3) 6(2,5)-5(2,4) 6(3,4)-5(3,3) 6(4,3)-5(4,2) 6(5,2)-5(5,1)
127685_9426(85) 128380.5241(115) 126989_1934(124) 127675.1786(75) 127670.0332(74) 127645_4978(88) 127607_7411(163) 127556.9841(258)
127682.3800(79) 128375.7096(117) 126986.8998(112) 127671.6739(60) 127666.5446(60) 127642.1181(74) 1276045152(148) 1275539389<233)
l@, I&-cm,
4(3,
0)
7(0,7)-6(O,6) 7(1,6)-6(1,5) 7(1,7)-6(1,6) 7(2,5)-6(2,4) 7(2,6)-6(2,5) 7(3,5)-6(3,4) 7(44,4)-6(4,3) 7(5.3)-6(5.2)
148962.728 149774.717 148151.652
148962_7571(89) 149774_7266(121) 148151.6402(131) 148954.3230(82) 148946.0911(81) 148918.1178(96) 1488739498(176) 148814.6911(274)
-29.1 - 9.6 11.8
8(0,8)-7(O,7) 8(1,7)-7U.6) 8(1,8)-7(1,7) 8(2,6)-7(2,5) 8 (2.71-7 (296)
170237.630 171167.732 169312.963
170237.6449(91) 171167.7509(121) 1693129717(133) 170233.4428(87) 170221.0964(85)
-14.9 -18.9 - 8.7
170221.118
21.6
148958586 148148.980 148950.205 148942.049
170232.902 171161.342 169309.907 170228.766 170216.468
ohs.- talc. Wz)
148958.6098(83) 149769.1141<124) 148148_9660(119) 148950.2287(65) 148942.0228(65) 148914_1744(81) 148870.1857(159) 148811.1376(248)
-23.8
1702329172(84) 171161.3423(124) 1693099176<121) 170228_7565(70) 170216.4490(70)
-15.2 - 0.3 -10.6 95 19.0
14.0 -23.7 26.2
(COntinuedonthene :xtpage) 382
Transition
Jrk_a,K&-J(~~,&)
6inversionstatefreq~encies(Itftrz) O-inversion state frequencies(MHz) -_ abs.- cak. observed calcuiated observed calculated W-W
170190.004 1b) 8&S)-7(3,4) 1~0190_004 8(3,6%-7(3,S) 8f4,fF-7(4,4) 8(5,4)---7(5,3) l~(O,lOl-9(~,9~ 10~1,9~-9(1,8) 213949.625 10
2L278Q5421(9f) 2139495892(108) 211631.6538<120) 2127925922~100) 212767 3478(97) 21273~.0724~~i~~ 212?31.~273(110) 212667.2733(i78) 212582,3822(249)
11(0,X1)-lO(0, X0) 234047.977 l~~l,~O)-lO(i.9) 235338.079 l1(1,11,-lO(r,lo~ 232788.700 11(2,9)-lO(Z,S) 234070.607 11(2,10)-10(2,9) 23403830.5 11(3,8)--iO(3,7) ~~~~~~~~~b) 11(3,9)-10(3,8) il{4,8)-10(4,7) 233929578 233836.055 116,7)-10(5,6)
234048.0029(98) 235338.0652(100) 232788.6880(111) 234070.6130(112) 234038.2934(109) 234000.0587(115) 233999.9854(1153 2339295626(171) 233836.0756<2183
13(0,13)--12CQ,t2~ 13(1,12)--12(1,Xlf278109.287 f3f1,23)-12Cl,i2) 275097.369 13(2,Xl)-12(2,10) 13(2,12)--12(2,11) 13(3,IO)-12(3,9) 13(3,11)-12<3,LO) 13(4,fO)-l2(4,9) 13(5,9)-1265,s)
276573.5807(~53~ 278109.2834tX2S) 275097_3S39(1273 2766285224(163) 276575.0733(160) 276534.401361&3) 276534.2303(142) 276450.221311745 276339.4485(177)
14~0,14)--13(0,13)29783X.143 14C1,13%--13C1,12~2P9491.700 1411,14)-13(1,13) 296248.685 14(2,X&--13(2,1X) 297907.385 14t2,IJ)---13(2,12) 297840580 14(3,11)--13(3,10)297799.407 14<3,12)--13(3,1X)297799.407 k) 14f4,11)-13(4,10) 297708.352 14(S,10)-13tS,P) 297588.907
297831,1520(204) -9.0 29949~.6~60(171) 14.0 296248,671f(172) 13.9 297907.3995(204) -14.5 "29784~_6~73(203} -27.3 297799~46?~~72) -139-7 2977P9_2P~~(~7~~ 109.0 297708.3600(204) -8.0 297588.8917(227) 15.3
17018V.PPP3C102) 170~8P.P8SO(lO2~ f70i39.3516(183) 170071.57l9(278)
27018S.4918@6) ~7018S_477S~86) 170135.049161653 f70067.5095(252)
4.7 19.0
35.8
-25.9 13.8 120 -60 11.6 -46.7 26.6 15 4 -20.6 , 3.6 15.1
obs-- c&c. w-w
213941.594
2127746683(84) 21394159SS~lOP~ - 18 2116278428(113) 212785.7133(82; 212?615451[82~ 212725.4359(93) 212725395Oc93) 222661.89336161) 212577.3008(226)
234041.5647(90) 235329.283469) 232784.5001(108) 234064.1327(95) 234031.9146(95) 233993.857269) ~~~~~~-~:~~b~~33993_7~42(9~) 2339231649 233923.6433(154) x33820.4928(197) 233830.472 234041.534 235329.306 232784520 234064-129 234031.918
278098.941
297823.060 299480.570 296243.376 297899.080 237832.485 297791504 297791.509=I 297700.822 297581,773
1.2 1.55
276S660338(140) 278098.93481113) 27SO92.4158(135)= 2766208275(14SE 276567.5458(14S) 276527_0686(127) 276526.8983~~26) 2764432224(155) 276332.834OCl60)
-30.7 22.6 19-P - 3.7 3-4 -47.2 25.8 5.7 -11.8 62
2978230618B86) -1.8 299480_5594(157) 10.6 296243.3600ff79) 16.0 297899.0908(184) -10.8 2978325076(184) -22.6 2977916478fEGi -1358 29779~.4000(~53) 109.0 297700.8207(182) l"3 297581.7648(205) 8.2
a)Numers,inparenthesessarestandarddeviatioas. b) Unre~lv~~~=3doubIetlineusedinM, ~~Ulresol;edKa=3doubi~tlinenatuse&~fit, informationtharthebaniertoin~~r~unisretati~ in turn shouldrest& high,namely ~2000crrr1.Th.i 7 iuaconsiderablysmallerinversion splittixtgthanwas
founctinthecyauamidespectnrm,w~erethebarrier hei~ti~~~y~7~3O~crn-~ 1121.The assigumentof theobservedspectrumto the moLcuIeisocyanamide
383
Volume 81, number 2
CHJZWCAL
PHYSICS LETTERS
was therefore supported by the appearance of nearby satellite lines shown in fig. 1, arising from molecules in the first-excited H2N-inversion state. Similar doublets, but more widely separated, were observed and analyzed in the microwave and millimeter-wave spectrum of H2N-CN [S-l 1 ] _ Due to the H2N-inversion motion the pyramidal isocyanamide behaves almost like a C,, molecule_ The vrbrational states of the H,N inversion motion with even u will thus possess statistical weights of 1 for rotational energy levels with even I$ and 3 for levels with odd K,. For transitions arising from the vrbrational states with odd u the statistical weights are reversed [S] _ This effect can be clearly seen in fig. 1 _ The two vibrational states with u = 0 and u = 1 are labeled in accordance with the rotation-inversion theory of Attanasio and co-workers [9,10] as O+ and Ostates, respectively. The preliminary analyses include 26 measured lines arising from the O+ inversion state and 27 lines arising from the O- state. From these data the rotatronal constants and centrifugal distortion constants were determined using the extended Watson S-reduced
Table 2
Spectral parameters of isocyanamidc for Watson’s S-reduced hamtltonian m Ir axis representation _ ______--~ 0’ inversion state O- mversron state Parameter ___-_--_____ 282 152(140)“) 282 139(130) A (MHZ) 10 757.2791(22) 10 525.338?(23)
B (MHz) C(~lilZ)
5 2845(27) 424.36 (37) -O-1776(33) -0 0323(21)
0~ (ktlz) DJK (kHz) rlt &HZ) & (kHZ) trjn (Hz) ffK_f UiZ) LK_I(Hz)
4 33 (37) --516(39) 18.35 (100)
10 756.7698
5.2815 (24) 422.44 (33) -0.1739 (33) -0.0309 (19) 4.15(33) --516(35) 18.54(87)
number of equally wetghted lines ip fit: 28 standard devtatton of fit (kHz) ---_____ a) Numbers
384
24 2
(22)
10 525.2509(21)
29
____
in parentheses are standard errors.
21.9
hamiltonian 9l=~(B+C)P~+
15 July 1981
[23], [A-;((B+c)]P!
where?, kX, f” and ?* are the operators for the total angular momentum and its components and ~~ =p_Y + i?,, . The coefficients are Watson’s reduced rotational constants A, 8, and C rn the 1’ axis representation. The adjusted constants are given in table 2. The Standard deviation of the fit is only slightly larger than the estimated error of the individual measurements. The adjusted constants allow precise frequency predictions of the a-type R-branch transitions in the microwave and millimeter-wave region which are also given in table 2.
4. Discussion From the above results as summarized in table 3, it is concluded that the microwave spectrum of the most abundant isotoprc species of isocyanamide has been detected. Until data from isotopically substituted species become available we can only confirm that the ab initio geometry of the molecule presented by Vincent and Dykstra 13 ] (see table 3) is the most reasonable structure of isocyanamide. The relatively high stability observed for the molecule is also in accord with their calculations. The relative intensities of the rotational transitions arising from the O+ and O- states are found to be 3 I 1 within experimental accuracy. No intensity difference due to the difference in energy of the two inversion states could be detected. Furthermore, no perturbations due to Coriolis-type interactions such as found for cyanamide were observed in the isocyanamide spectrum for either state up to K, = 5. However, the need of the higher-order L, centrifugal distortion term in fitting the spectral lines was observed for both states. From these facts and from the close inversion doublets we infer that the splitting between the inversion states Of and O- is probably less than 10 cm-l. The data now available for isocyanamide should
Volume 8 1, number 3
CHEMICAL PHYSICS LETTERS
-
15 July 1981
Table 3 Compa.&on of spectroscopic constants for isocyanamide with different ab initio structures From ab-irdtio
Rotational constants
caIcuiations
ref. [I] -
I? (MHZ) C(MHz) B -I-C (MHz) B - C (MHz)
A (MHz)
260 040 10 167 9 873 20 040 294
ab imtlo structures
(SCF MO)
r(NC)
(A)
~zoob)
1,420 1.030 1265 127.5 180
according
to structures
ref. [2] ____A_
ref. [3]
Lhperimental this work -
average of O+, O- states -_~-~
279 700 11086 10 854 21940 232
294 446 a) 10 877 10 644 21521 233
(631G)
(SCEP)
1.164
1.169 a? 1.352 0997 111.4 124 7 1769
1.345
values
282 146 10 757.024 10535 294 21282.318 231 73
U^
0.99A 113.4 =) WNW @eg) L (NN-NH2 pIane) (deg) 116 5 =) 180 L (NNCI (de& -- _-----a) The presentation of the reported structure ISnot umqueiy pven; furthermore the rotational constants reported cannot be reproduced if the structure is interpreted as given here. b) Ab initio value corrected empiricallyfrom 1.250 to 1 200 A. C) The presentation of the reported anglesis assumed to be in error and interpreted as given here. L
be sufficiently accurate to identify it via its rotational spec:rum as a constituent in interstellar clouds if the proposed ion-molecule reaction sequence {3f H,N+ f NC + H,NNC’s
H,NNC + H’ ,
facilitates the formation of the molecule without too much internal energy. The excess internal energy would be removed by the hydrogen atom ejected in the recombination reaction, thus stabilizing H,N-NC.
Acknowledgement One of as (JJC) expresses his gratitude to -&e Physikalisch-Chemisches Instltut der Justus LiebigUniversitlt Giessen for hospitahty extended to him during the summer months of 1980 when this work was initiated_ The authors express their thanks to Dr. Brenda P. W~new~ser for many discussions and critically reading and commenting on the manuscript. The work was supported in part by funds from the Deutsche Forschungsgemeinschaft, the Max-Planck Institut fQr Radioastronomie and the Fonds der Chemischen Industrie.
References [i] B.T. Hart, Australian J. Chem 76 (1973) 461. (21 J-B. Moffat, J. hfol. Struct. 52 (1979) 275. [3] M.A. Vincent and C.C. Dykstra, J. Chem. Phys 73 (1980) 3838. [4] J-K. Tyler, L.F. Thomas and J. Sheridan, Proc Chem. Sot. (1959) 155. [S] D-J. Millen, G_ Topping and D.R. Lrde Jr_, 3. Mol. Spectry. 8 (1962) 153. [6] D.R. Lide Jr., J. Mol. Spectry. 8 (1962) 142 [7] J.N. MacDonald, D. Taylor, J-K. Tyler and J. Sheridan, J. Mol. Spew-y. 26 (1968) 285. [S] J-K. Tyler, J_ Sheridan and CC. Costam, J. Mol. Spectry. 43 (1972) 248. [9] A. Attanasio. A. Bauder, Hs H. Gil&hard and H J. Keller, Mel_ Phys. 21(1971) 35_ [lo] A. Attanasio, A. Bauder and Hs.H. Cilnthard, Chem. Phys. 6 (1974) 373. [ 1 l] D-R. Johnson, R.D. Suenram and W J. Lafferty, Astrophys. J. 208 (1974) 24.5. [lZ] T.R Jones and N. Sheppard, Chem. Commun. (1970) 71.5. [ 13 j B.E. Turner, A-G. Kislyakov, H.S. Lxszt and N. Kaifu, Astrophys. J. 201(1975) Li49. [14] P.G. Wannier and R-A. Linke, Astrophys. 3. 226 (1978) 817. [IS] E. MBller and D. Ludsteck, Chem. Ber. 87 (1954) 1887.
385
Volume
15 July 1981
CHEMICAL PHYSICS LETTERS
8 1, number 2
[16] E. hliiller, P. IGistner and W. Rundel, Chem. Ber. 98 (1965) 711 [17] E. hIfiller, P. KSstner, R. Beutler, W. Rundel, H. Suhr and B. Zeeh, Liebigs Ann Chem. 713 (1968) 87.
[19] J-F. Ogllvie, J. Mol. Struct. 3 (1969) 513. 1201 J--P. Anselme, J. Chem. Educ. 54 (1977) 296. [21] E. Schtfer and M. Wirmewisser, to be published. [22] [23]
[ 181 E. Miller, R. Beutler and B. Zeeh, Liebigs Ann. Chcm 7 19 (1968) 72.
E. MUler and W. Rundel, Chem. Ber. 90 (1957) 1302. J KG. Watson, III: Vibrational spectra and structure, ed. J.R. Durlg (Elsevier, Amsterdam, 1977)
ERRATA S.W. Haan and L-R. Pratt, Monte Carlo study of a simple mode1 for nacelle structure. Chem. Phys. Letters 79 (1981) 436. An error has been made in transcnbing calculated results to table 1. table ! is shown below. The only change is for the relative mean-square length of the molecules in the aggregates,
Table 1
N=20 u/_vkBT
10) a) fi*) a) fi3>
a)
w&IN nthfN nttrN SjN b) Sh/N b) tr*)/tr*)e
c)
-7.64 k 3.35 x 1.76 X 1.10 x 0.017 0 250 2.13 5.21 2.72 1.03
0.02 102 lo* 102
N=30
N=50
-7.90 k 0.01 6.95 x lo* 3.51 x 102 2.07 x lo* 0.024 0.328 2.23 4.84 2.62 1.03
-8.04 3.29 6.45 4.32 0.035 0.368 2.28 4.63 2 56
r x X X
0.01 103 lo* lo*
1.03
R-S. Karve, SX. Sarkar, K.V.S. Rama Rao and JP. Mittal, Sensitized multiphoton dissociation of UF, in SF,-UF, mixtures by a TEA CO2 laser, Chem. Phys. Letters 78 (198 1) 273_ The equation for intermode PTW-V) SF&3)
< 150
relaxation
should read:
nsTorr l
SF;(+)
-
S. Emid, Existence of spin symmetry species in reorienting methyl groups at high temperatures, Chem- Phys. Letters 80 (1981) 393. The name of Professor Smidt was misspelled in the acknowledgement: it should read J. Smidt, not J. Schmidt. 386
8 1, number
2
MILLIMETER-WAVE
E&hard
SCH&ER,
CHEMICAL
PHYSICS
SPECTRUM OF ISOCYANAMIDE,
LETTERS
15 July
1981
HzN-NC.
Manfred WINNEWlSSER
Physikalisch-Chemisches Institut. Justus-Liebig-UniversZt
Giessen. D-6300
Giessen, West Germany
and J@rn Johs. CHRISTIANSEN Department of Chemistry, The Royal Danish School of Educational Studies. DK-2400 Received
n
6 March 1981;
in final form 19 March 1981
The a-type rotatronal spectrum of isocyanamide was observed of the hydrolysis products of diazomethj llithmm was employed signed to molecules in theground vibrational state (NH&wersion (NHZ-inversion state 03.
l_ Introduction
Several theoretical studies of the various diazomethane Isomers [l-3] have predicted that cyanamide (I) is the most stable isomer followed by isocyanamide (II) and diazomethane (III). H,N-CN I
H,N-NC II
Copenhagen NV. Denmark
H,C=N=N III
HC=N=NH IV
has been the subject of numerous spectroscopic investigations [4-121 and its molecular structure and H2N-inversion motion have been analyzed_ Millimeter-wave measurements [ 111 confirmed the identification of cyanamide as a constituent of the interstellar molecular cloud in Sgr B2 [ 13,14]_ Isocyanamide was prepared by Milller and Ludsteck _[ 151 but was mistaken for nit&mine (IV)_ However, the chemical reactions of the compound which were carried out later provided strong evidence for isocyanamide [ 16-18]_ The isocyanamide structure was also supported by infrared studies [ 171 in solution and ti the solid phase [ 19,20]_ NMR spectra [17,20] and the strong isonitrile smell of the hydrolysis products of LiCHN, ) observed by MIiller and Ludsteck [ 151, further supported structure II. The barrier to isomerization between the structures Cyanamide
in the frequency range 147-300 GHz. Ether extraction to isolate the isocyanamide. 53 rotational lines were asstateO+) and m the lowest excited vtbrational state
I and II was reported by a recent ab initio calculation to be of the order of 46 kcal/mol which should stabilize the isocyanamide molecule at moderate temperatures [3] _Another pathway of isomerization from structure IV might be possible but has not been calculated_ In this paper we describe our results in searching for the isocyanamide millimerer-wave rotational spectrum and its unambiguous assignment. During the course of this work the millimeter-wave spectrum of diazomethane was also measured in the entire millimeter-wave range. The diazomethane data will be reported at a later date [21] _
2. Experimental procedures Isocyanamide was prepared dccordiug to the method described by Miiller and Ruqdel [22]. A solution of N20 in ether at a temperature of -85°C was treated with an excess of methyl lithium. The precipitate was separated by filtration and afterwards suspended in ether. This suspension was hydrolyzed with an aqueous solution of KH2P04 _Isocyanamide was extracted with ether, dried and concentrated in high vacuum. At a temperature of -30°C the sample was pumped for several hours until the vapour pressure remained below 2 pbar.
380 0 009-26
14/8 1 /OOOO-0000/S
02-50 0 North-Holland Publishing Company
CHEMICAL PHYSICS LETTERS
Volume 81, number 2
The search for the millimeter-wave rotational spectrum was performed with a newly built submillimeter-wave spectrometer designed for the study of unstable species 1211. The absorption cell consisted of a Pyrex free-space cell with an inner diameter of 10 cm and a length of 150 cm. The spectrum was measured using frequency multiplication from a klystron fundamental frequency between 60 and 80 GHz and employing source modulation and second-harmonic phase-sensitive detection of the modulation frequency of 3.5 kHz. Fig. 1 shows a survey spectrum of one transition recorded with this system. The absorption cell was fdled with isocyanamide vapour by keeping the sample at a temperature of -25°C and evaporating it continuously into the cell. The half-life of isocyanamide in the absorption cell at a total pressure of 10 pbar is = 15 min which is comparable to the stability observed for diazomethane under similar conditions. The partial pressure of isocyanamide probably did not exceed 4 pbar. Cyanamide has been identified as a by-product of the abovementioned synthesis or as a decomposition product via Its known millimeter-wave spectrum [ 1 l] _ Diazomethane was
15 July 1981
not observed as a decomposition product of isocyanamide. It should be mentioned that in a gold-plated Stark cell of a conventional microwave spectrometer, isocyanamide could not be observed due to rapid decomposition.
3. Spectra and preliminary analysis The observed transitions found in the millimeterwave region showed the typical pattern of a-type Rbranch lines with almost equidistant Ka = 0 lines for successive J values and widely spread K, = 1 doublets due to the slight asymmetry of the molecule (see table 1). The observed i (B + C) and B - C values indicated strongly that the observed lmes arise from isocyanamide. The rotational constants are in very good agreement with the latest ab initio calculations reported by Vincent and Dykstra [3 J _In particular, they predicted the B - C value to be 233 MHz, and the average B - C for the O+ and O- states found from our results is 23 1.7 MHZ. The ab initio calculations [3] provided the further
H,N-NC
850
233)900
950
234000
MHZ
Fig. l_ Part of the observed millimeter-wave spectrum of isocyanamide at 234 GHz with the typical Ka pattern for both inversion states O+ and O- exh~biiing the effect of nuclear spin statistics.
381
Volume81,numberZ
CHEMICALPHYSICSLETTERS
lSJuly1951
Table 1 a-typeR-branchrotationaltransitionsofisocyanamide,HtN-NC --Transttion JW,, Kc+-JCK,,
O'inversion statefrequencies(MHz)
O-inversionstate frequencies(MHz)
&I observed
calculated --
obs -talc. observed &Hz)
calculated
212825969(18)a)
212819996 (17)
2(0,2)-l(O,l) 2(1,1)-l(1,O) 2(1.2)-l(l,l)
42564.9183(36) 42795.3019(48) 42331.4330(51)
42563.7244(33) 42793.6940(49) 42330.6673(46)
3(0,3)-2(0,2) 3(1,2)-2(1,1) 3(1,3)-2(1,2) 3(2,1)-2(2,0) 3(2,2)-2(2,1)
63846.6890(52) 641925325(70) 63496.7506(74) 638376351(44) 63837.0469(44)
63844.8995(48) 64190.1214(71) 63495.6022(67) 63835.8875(35) 63835.3012(35)
4(0,4)-3(0,3) 4(1,3)--3(J,2) 4(1,4)-3(1,3) 4(2,2)-3(2,1) 4(2.3)-3(2,2) w--3(3,1)
85127.6334(66) 85589.2586(89) 84661.5894(95) 851168311(56) 85115.3601\56) 85098_4304(65)
85125.2503(61) 85586_0451(90) 84660.0588(86) 85114_4993(45) 85113.0336(445) 85096.1777(55)
5(0,5)-4(0,4) 5(1,4)-4(1,3) 5(1,5)-4(1,4) 5(2,3)-4(2,2) 5(2,4)-4(2,3) 5(3,3)-4(3,2) 5(4,2)-4(4,1)
106407.4763(77) 106985.3119(104) 105825.7901(112) 106396_0134(67) 106393.0729(66) 106372.2327(77) 106340 8408(145)
106404.5020(72) 1069812972(106) 105823.8777(101) 106393.0960(53) 106390.1648(53) 106369.4166(65) 106338.1529(131)
6(0,6)--5(O,5) 6(1,5)--5(1,4) 6(L,6)-5(1,5) 6(2,4)-S(2,3) 6(2,5)-5(2,4) 6(3,4)-5(3,3) 6(4,3)-5(4,2) 6(5,2)-5(5,1)
127685_9426(85) 128380.5241(115) 126989_1934(124) 127675.1786(75) 127670.0332(74) 127645_4978(88) 127607_7411(163) 127556.9841(258)
127682.3800(79) 128375.7096(117) 126986.8998(112) 127671.6739(60) 127666.5446(60) 127642.1181(74) 1276045152(148) 1275539389<233)
l@, I&-cm,
4(3,
0)
7(0,7)-6(O,6) 7(1,6)-6(1,5) 7(1,7)-6(1,6) 7(2,5)-6(2,4) 7(2,6)-6(2,5) 7(3,5)-6(3,4) 7(44,4)-6(4,3) 7(5.3)-6(5.2)
148962.728 149774.717 148151.652
148962_7571(89) 149774_7266(121) 148151.6402(131) 148954.3230(82) 148946.0911(81) 148918.1178(96) 1488739498(176) 148814.6911(274)
-29.1 - 9.6 11.8
8(0,8)-7(O,7) 8(1,7)-7U.6) 8(1,8)-7(1,7) 8(2,6)-7(2,5) 8 (2.71-7 (296)
170237.630 171167.732 169312.963
170237.6449(91) 171167.7509(121) 1693129717(133) 170233.4428(87) 170221.0964(85)
-14.9 -18.9 - 8.7
170221.118
21.6
148958586 148148.980 148950.205 148942.049
170232.902 171161.342 169309.907 170228.766 170216.468
ohs.- talc. Wz)
148958.6098(83) 149769.1141<124) 148148_9660(119) 148950.2287(65) 148942.0228(65) 148914_1744(81) 148870.1857(159) 148811.1376(248)
-23.8
1702329172(84) 171161.3423(124) 1693099176<121) 170228_7565(70) 170216.4490(70)
-15.2 - 0.3 -10.6 95 19.0
14.0 -23.7 26.2
(COntinuedonthene :xtpage) 382
Transition
Jrk_a,K&-J(~~,&)
6inversionstatefreq~encies(Itftrz) O-inversion state frequencies(MHz) -_ abs.- cak. observed calcuiated observed calculated W-W
170190.004 1b) 8&S)-7(3,4) 1~0190_004 8(3,6%-7(3,S) 8f4,fF-7(4,4) 8(5,4)---7(5,3) l~(O,lOl-9(~,9~ 10~1,9~-9(1,8) 213949.625 10
2L278Q5421(9f) 2139495892(108) 211631.6538<120) 2127925922~100) 212767 3478(97) 21273~.0724~~i~~ 212?31.~273(110) 212667.2733(i78) 212582,3822(249)
11(0,X1)-lO(0, X0) 234047.977 l~~l,~O)-lO(i.9) 235338.079 l1(1,11,-lO(r,lo~ 232788.700 11(2,9)-lO(Z,S) 234070.607 11(2,10)-10(2,9) 23403830.5 11(3,8)--iO(3,7) ~~~~~~~~~b) 11(3,9)-10(3,8) il{4,8)-10(4,7) 233929578 233836.055 116,7)-10(5,6)
234048.0029(98) 235338.0652(100) 232788.6880(111) 234070.6130(112) 234038.2934(109) 234000.0587(115) 233999.9854(1153 2339295626(171) 233836.0756<2183
13(0,13)--12CQ,t2~ 13(1,12)--12(1,Xlf278109.287 f3f1,23)-12Cl,i2) 275097.369 13(2,Xl)-12(2,10) 13(2,12)--12(2,11) 13(3,IO)-12(3,9) 13(3,11)-12<3,LO) 13(4,fO)-l2(4,9) 13(5,9)-1265,s)
276573.5807(~53~ 278109.2834tX2S) 275097_3S39(1273 2766285224(163) 276575.0733(160) 276534.401361&3) 276534.2303(142) 276450.221311745 276339.4485(177)
14~0,14)--13(0,13)29783X.143 14C1,13%--13C1,12~2P9491.700 1411,14)-13(1,13) 296248.685 14(2,X&--13(2,1X) 297907.385 14t2,IJ)---13(2,12) 297840580 14(3,11)--13(3,10)297799.407 14<3,12)--13(3,1X)297799.407 k) 14f4,11)-13(4,10) 297708.352 14(S,10)-13tS,P) 297588.907
297831,1520(204) -9.0 29949~.6~60(171) 14.0 296248,671f(172) 13.9 297907.3995(204) -14.5 "29784~_6~73(203} -27.3 297799~46?~~72) -139-7 2977P9_2P~~(~7~~ 109.0 297708.3600(204) -8.0 297588.8917(227) 15.3
17018V.PPP3C102) 170~8P.P8SO(lO2~ f70i39.3516(183) 170071.57l9(278)
27018S.4918@6) ~7018S_477S~86) 170135.049161653 f70067.5095(252)
4.7 19.0
35.8
-25.9 13.8 120 -60 11.6 -46.7 26.6 15 4 -20.6 , 3.6 15.1
obs-- c&c. w-w
213941.594
2127746683(84) 21394159SS~lOP~ - 18 2116278428(113) 212785.7133(82; 212?615451[82~ 212725.4359(93) 212725395Oc93) 222661.89336161) 212577.3008(226)
234041.5647(90) 235329.283469) 232784.5001(108) 234064.1327(95) 234031.9146(95) 233993.857269) ~~~~~~-~:~~b~~33993_7~42(9~) 2339231649 233923.6433(154) x33820.4928(197) 233830.472 234041.534 235329.306 232784520 234064-129 234031.918
278098.941
297823.060 299480.570 296243.376 297899.080 237832.485 297791504 297791.509=I 297700.822 297581,773
1.2 1.55
276S660338(140) 278098.93481113) 27SO92.4158(135)= 2766208275(14SE 276567.5458(14S) 276527_0686(127) 276526.8983~~26) 2764432224(155) 276332.834OCl60)
-30.7 22.6 19-P - 3.7 3-4 -47.2 25.8 5.7 -11.8 62
2978230618B86) -1.8 299480_5594(157) 10.6 296243.3600ff79) 16.0 297899.0908(184) -10.8 2978325076(184) -22.6 2977916478fEGi -1358 29779~.4000(~53) 109.0 297700.8207(182) l"3 297581.7648(205) 8.2
a)Numers,inparenthesessarestandarddeviatioas. b) Unre~lv~~~=3doubIetlineusedinM, ~~Ulresol;edKa=3doubi~tlinenatuse&~fit, informationtharthebaniertoin~~r~unisretati~ in turn shouldrest& high,namely ~2000crrr1.Th.i 7 iuaconsiderablysmallerinversion splittixtgthanwas
founctinthecyauamidespectnrm,w~erethebarrier hei~ti~~~y~7~3O~crn-~ 1121.The assigumentof theobservedspectrumto the moLcuIeisocyanamide
383
Volume 81, number 2
CHJZWCAL
PHYSICS LETTERS
was therefore supported by the appearance of nearby satellite lines shown in fig. 1, arising from molecules in the first-excited H2N-inversion state. Similar doublets, but more widely separated, were observed and analyzed in the microwave and millimeter-wave spectrum of H2N-CN [S-l 1 ] _ Due to the H2N-inversion motion the pyramidal isocyanamide behaves almost like a C,, molecule_ The vrbrational states of the H,N inversion motion with even u will thus possess statistical weights of 1 for rotational energy levels with even I$ and 3 for levels with odd K,. For transitions arising from the vrbrational states with odd u the statistical weights are reversed [S] _ This effect can be clearly seen in fig. 1 _ The two vibrational states with u = 0 and u = 1 are labeled in accordance with the rotation-inversion theory of Attanasio and co-workers [9,10] as O+ and Ostates, respectively. The preliminary analyses include 26 measured lines arising from the O+ inversion state and 27 lines arising from the O- state. From these data the rotatronal constants and centrifugal distortion constants were determined using the extended Watson S-reduced
Table 2
Spectral parameters of isocyanamidc for Watson’s S-reduced hamtltonian m Ir axis representation _ ______--~ 0’ inversion state O- mversron state Parameter ___-_--_____ 282 152(140)“) 282 139(130) A (MHZ) 10 757.2791(22) 10 525.338?(23)
B (MHz) C(~lilZ)
5 2845(27) 424.36 (37) -O-1776(33) -0 0323(21)
0~ (ktlz) DJK (kHz) rlt &HZ) & (kHZ) trjn (Hz) ffK_f UiZ) LK_I(Hz)
4 33 (37) --516(39) 18.35 (100)
10 756.7698
5.2815 (24) 422.44 (33) -0.1739 (33) -0.0309 (19) 4.15(33) --516(35) 18.54(87)
number of equally wetghted lines ip fit: 28 standard devtatton of fit (kHz) ---_____ a) Numbers
384
24 2
(22)
10 525.2509(21)
29
____
in parentheses are standard errors.
21.9
hamiltonian 9l=~(B+C)P~+
15 July 1981
[23], [A-;((B+c)]P!
where?, kX, f” and ?* are the operators for the total angular momentum and its components and ~~ =p_Y + i?,, . The coefficients are Watson’s reduced rotational constants A, 8, and C rn the 1’ axis representation. The adjusted constants are given in table 2. The Standard deviation of the fit is only slightly larger than the estimated error of the individual measurements. The adjusted constants allow precise frequency predictions of the a-type R-branch transitions in the microwave and millimeter-wave region which are also given in table 2.
4. Discussion From the above results as summarized in table 3, it is concluded that the microwave spectrum of the most abundant isotoprc species of isocyanamide has been detected. Until data from isotopically substituted species become available we can only confirm that the ab initio geometry of the molecule presented by Vincent and Dykstra 13 ] (see table 3) is the most reasonable structure of isocyanamide. The relatively high stability observed for the molecule is also in accord with their calculations. The relative intensities of the rotational transitions arising from the O+ and O- states are found to be 3 I 1 within experimental accuracy. No intensity difference due to the difference in energy of the two inversion states could be detected. Furthermore, no perturbations due to Coriolis-type interactions such as found for cyanamide were observed in the isocyanamide spectrum for either state up to K, = 5. However, the need of the higher-order L, centrifugal distortion term in fitting the spectral lines was observed for both states. From these facts and from the close inversion doublets we infer that the splitting between the inversion states Of and O- is probably less than 10 cm-l. The data now available for isocyanamide should
Volume 8 1, number 3
CHEMICAL PHYSICS LETTERS
-
15 July 1981
Table 3 Compa.&on of spectroscopic constants for isocyanamide with different ab initio structures From ab-irdtio
Rotational constants
caIcuiations
ref. [I] -
I? (MHZ) C(MHz) B -I-C (MHz) B - C (MHz)
A (MHz)
260 040 10 167 9 873 20 040 294
ab imtlo structures
(SCF MO)
r(NC)
(A)
~zoob)
1,420 1.030 1265 127.5 180
according
to structures
ref. [2] ____A_
ref. [3]
Lhperimental this work -
average of O+, O- states -_~-~
279 700 11086 10 854 21940 232
294 446 a) 10 877 10 644 21521 233
(631G)
(SCEP)
1.164
1.169 a? 1.352 0997 111.4 124 7 1769
1.345
values
282 146 10 757.024 10535 294 21282.318 231 73
U^
0.99A 113.4 =) WNW @eg) L (NN-NH2 pIane) (deg) 116 5 =) 180 L (NNCI (de& -- _-----a) The presentation of the reported structure ISnot umqueiy pven; furthermore the rotational constants reported cannot be reproduced if the structure is interpreted as given here. b) Ab initio value corrected empiricallyfrom 1.250 to 1 200 A. C) The presentation of the reported anglesis assumed to be in error and interpreted as given here. L
be sufficiently accurate to identify it via its rotational spec:rum as a constituent in interstellar clouds if the proposed ion-molecule reaction sequence {3f H,N+ f NC + H,NNC’s
H,NNC + H’ ,
facilitates the formation of the molecule without too much internal energy. The excess internal energy would be removed by the hydrogen atom ejected in the recombination reaction, thus stabilizing H,N-NC.
Acknowledgement One of as (JJC) expresses his gratitude to -&e Physikalisch-Chemisches Instltut der Justus LiebigUniversitlt Giessen for hospitahty extended to him during the summer months of 1980 when this work was initiated_ The authors express their thanks to Dr. Brenda P. W~new~ser for many discussions and critically reading and commenting on the manuscript. The work was supported in part by funds from the Deutsche Forschungsgemeinschaft, the Max-Planck Institut fQr Radioastronomie and the Fonds der Chemischen Industrie.
References [i] B.T. Hart, Australian J. Chem 76 (1973) 461. (21 J-B. Moffat, J. hfol. Struct. 52 (1979) 275. [3] M.A. Vincent and C.C. Dykstra, J. Chem. Phys 73 (1980) 3838. [4] J-K. Tyler, L.F. Thomas and J. Sheridan, Proc Chem. Sot. (1959) 155. [S] D-J. Millen, G_ Topping and D.R. Lrde Jr_, 3. Mol. Spectry. 8 (1962) 153. [6] D.R. Lide Jr., J. Mol. Spectry. 8 (1962) 142 [7] J.N. MacDonald, D. Taylor, J-K. Tyler and J. Sheridan, J. Mol. Spew-y. 26 (1968) 285. [S] J-K. Tyler, J_ Sheridan and CC. Costam, J. Mol. Spectry. 43 (1972) 248. [9] A. Attanasio. A. Bauder, Hs H. Gil&hard and H J. Keller, Mel_ Phys. 21(1971) 35_ [lo] A. Attanasio, A. Bauder and Hs.H. Cilnthard, Chem. Phys. 6 (1974) 373. [ 1 l] D-R. Johnson, R.D. Suenram and W J. Lafferty, Astrophys. J. 208 (1974) 24.5. [lZ] T.R Jones and N. Sheppard, Chem. Commun. (1970) 71.5. [ 13 j B.E. Turner, A-G. Kislyakov, H.S. Lxszt and N. Kaifu, Astrophys. J. 201(1975) Li49. [14] P.G. Wannier and R-A. Linke, Astrophys. 3. 226 (1978) 817. [IS] E. MBller and D. Ludsteck, Chem. Ber. 87 (1954) 1887.
385
Volume
15 July 1981
CHEMICAL PHYSICS LETTERS
8 1, number 2
[16] E. hliiller, P. IGistner and W. Rundel, Chem. Ber. 98 (1965) 711 [17] E. hIfiller, P. KSstner, R. Beutler, W. Rundel, H. Suhr and B. Zeeh, Liebigs Ann Chem. 713 (1968) 87.
[19] J-F. Ogllvie, J. Mol. Struct. 3 (1969) 513. 1201 J--P. Anselme, J. Chem. Educ. 54 (1977) 296. [21] E. Schtfer and M. Wirmewisser, to be published. [22] [23]
[ 181 E. Miller, R. Beutler and B. Zeeh, Liebigs Ann. Chcm 7 19 (1968) 72.
E. MUler and W. Rundel, Chem. Ber. 90 (1957) 1302. J KG. Watson, III: Vibrational spectra and structure, ed. J.R. Durlg (Elsevier, Amsterdam, 1977)
ERRATA S.W. Haan and L-R. Pratt, Monte Carlo study of a simple mode1 for nacelle structure. Chem. Phys. Letters 79 (1981) 436. An error has been made in transcnbing calculated results to table 1. table ! is shown below. The only change is for the relative mean-square length of the molecules in the aggregates,
Table 1
N=20 u/_vkBT
10) a) fi*) a) fi3>
a)
w&IN nthfN nttrN SjN b) Sh/N b) tr*)/tr*)e
c)
-7.64 k 3.35 x 1.76 X 1.10 x 0.017 0 250 2.13 5.21 2.72 1.03
0.02 102 lo* 102
N=30
N=50
-7.90 k 0.01 6.95 x lo* 3.51 x 102 2.07 x lo* 0.024 0.328 2.23 4.84 2.62 1.03
-8.04 3.29 6.45 4.32 0.035 0.368 2.28 4.63 2 56
r x X X
0.01 103 lo* lo*
1.03
R-S. Karve, SX. Sarkar, K.V.S. Rama Rao and JP. Mittal, Sensitized multiphoton dissociation of UF, in SF,-UF, mixtures by a TEA CO2 laser, Chem. Phys. Letters 78 (198 1) 273_ The equation for intermode PTW-V) SF&3)
< 150
relaxation
should read:
nsTorr l
SF;(+)
-
S. Emid, Existence of spin symmetry species in reorienting methyl groups at high temperatures, Chem- Phys. Letters 80 (1981) 393. The name of Professor Smidt was misspelled in the acknowledgement: it should read J. Smidt, not J. Schmidt. 386